Relative humidity alone is a poor guide to how a plant feels, because the same percentage means different things at different temperatures. Vapour pressure deficit (VPD) fixes that by measuring the actual drying power of the air.
What VPD is
VPD is the difference, in kilopascals (kPa), between the amount of water vapour the air could hold at saturation and the amount it holds now. A higher VPD pulls water from the leaf faster; a lower one slows transpiration.
Saturation vapour pressure at temperature T (°C) can be estimated as 0.6108 × e^(17.27 T / (T + 237.3)) kPa. VPD is that value multiplied by (1 − RH/100).
A worked example
At 25 °C, saturation vapour pressure is about 3.17 kPa. At 70 % relative humidity the VPD is 3.17 × 0.30, about 0.95 kPa. Raise the temperature to 30 °C at the same humidity and VPD rises to roughly 1.3 kPa, so the plant transpires harder without any change in the humidity reading.
Typical targets
- Cuttings, seedlings and freshly hardened plants: low, about 0.4 to 0.8 kPa.
- Many crops in vegetative growth: about 0.8 to 1.2 kPa.
- Generative stages: often slightly higher, within the crop’s tolerance.
These are starting points. Each crop and stage has its own optimum, which is why a growth protocol sets VPD per stage.
Why it matters
- Too low: leaves stay wet, disease pressure rises and calcium moves poorly to new growth.
- Too high: stomata close to save water, photosynthesis drops and tips can burn.
How to control it
Temperature, humidity and airflow all move VPD. Vents, fans and screens lower temperature; fogging and misting raise humidity while also cooling; dehumidification and heating lower humidity at night. A controller that reads temperature and humidity together can act on VPD directly instead of on two separate set-points.
Our greenhouse design and control service builds this logic into the climate controllers and the control software. Related reading: PPFD and DLI and greenhouse irrigation control.